baffle step LOSS?

[madinoz]

Hope this doesn't ignite a flood of debate..........

The term "baffle step loss" has always confused me in it's use.
The term loss in my mind would equate to some acoustic reduction in output of a loudspeaker at the listening point.
I understand that a woofer (fo example) in a given box has a much lower level of bass output compared to midrange (6db).

If you take a lose look at any frequency response graph showing output on a baffle, the bass though lower than the midrange is still at the output of the driver's unbaffled response.

Take for example the Peerless HDS 6.5inch woofer.
Let's say it's characteristic sensitivity at 2.83volts/rms at 1 metre is 88db. The woofer is placed in a ported box which is tuned correctly and from say 400hz to 800hz it's output when 2.83 volts is applied is 88db. At the point after baffle step happens the response is 94db at the same voltage.

It seems rather unlogical to refer to this phenomenon as a "loss".
The listener (ie normal people who site in front of a speaker to listen to it not beside it), actually receives more acoustic output than if the woofer where unbaffled!!!

I know that there is a loss of acoustic output off-axis to the speaker i.e waves become directional, but since we have to assume that a listener in the near or far-field will be relatively on-axis to the speaker system then this would still not be percieved as a loss.

I propose to change the term to something like "baffle step directivity change" or something along those lines since it can be argued that there is actually no net increase or decrease of acoustic energy fro the whole system.

Just my two bobs worth.

MADINOZ

[sreten]

Checkout the Seas drivers measured in boxes, e.g. :

http://www.seas.no/seas_line/woofers/H571.PDF

the baffle step is obvious, the point is the measured bass
response is lower than it would be mounted on a large
"infinite" baffle , hence the term baffle step loss.

The bass level is 6dB below published sensitivity specs,
which is measured half field, not free field.

Your arguement regarding the Peerless woofer is incorrect.
If its sensisitivity is 88dB/2.83V this is above baffle step.
(As the driver is measured on a large baffle).
In the bass effective levels will be 82dB/2.83V.

sreten.

[madinoz]

The graph for the SEAS woofer has no absolute dB shown on the
dB scale therefore it is impossible to work out the absolute levels of output at any given frequency for a given voltage.

Please find attached a graph (copyright BESL) which shows a Vifa PL18 woofer in a Madisound box (from DIY2000).
You can note that the driver was placed into the speaker box and was when driven with a 2.83volt signal it exhibited a sensitivity of around 87/db/w/m which is basically Vifa's published sensitivity for this driver.
It's after the baffle step point is reached that the output rises to the +6db level.
You can view the full set of graphs at
http://www.t-linespeakers.org/tech/filters/Jeff/.

MADINOZ

[Svante]

Originally posted by madinoz
Hope this doesn't ignite a flood of debate..........

The term "baffle step loss" has always confused me in it's use.
The term loss in my mind would equate to some acoustic reduction in output of a loudspeaker at the listening point.
I understand that a woofer (fo example) in a given box has a much lower level of bass output compared to midrange (6db).

If you take a lose look at any frequency response graph showing output on a baffle, the bass though lower than the midrange is still at the output of the driver's unbaffled response.

Take for example the Peerless HDS 6.5inch woofer.
Let's say it's characteristic sensitivity at 2.83volts/rms at 1 metre is 88db. The woofer is placed in a ported box which is tuned correctly and from say 400hz to 800hz it's output when 2.83 volts is applied is 88db. At the point after baffle step happens the response is 94db at the same voltage.

It seems rather unlogical to refer to this phenomenon as a "loss".
The listener (ie normal people who site in front of a speaker to listen to it not beside it), actually receives more acoustic output than if the woofer where unbaffled!!!

I know that there is a loss of acoustic output off-axis to the speaker i.e waves become directional, but since we have to assume that a listener in the near or far-field will be relatively on-axis to the speaker system then this would still not be percieved as a loss.

I propose to change the term to something like "baffle step directivity change" or something along those lines since it can be argued that there is actually no net increase or decrease of acoustic energy fro the whole system.

Just my two bobs worth.

MADINOZ

The thing is... The sensitivity figures that are published are valid for an *infinite* baffle, or half space. At least those of Peerless, but also of most others. Decreasing the baffle size to that of an ordinary box, causes a *loss* of level in the bass. So, it's all a matter of selecting the viewpoint.
While we are at it; the effect is hardly a step function either. A step function goes *instantaneously* from one value to the other, that is how the matematicians define it anyway. The baffle "step" is very smooth.
So, Id suggest "finite baffle bass loss" or something, if I had to. But really, everyone knows what the baffle step is, so why confuse it?

BTW, what does EMF mean?

[planet10]

Jeff's chart seems to be bumped in level...here is the peerless chart

dave

[sreten]

Originally posted by madinoz
The graph for the SEAS woofer has no absolute dB shown on the
dB scale therefore it is impossible to work out the absolute levels of output at any given frequency for a given voltage.

Please find attached a graph (copyright BESL) which shows a Vifa PL18 woofer in a Madisound box (from DIY2000).
You can note that the driver was placed into the speaker box and was when driven with a 2.83volt signal it exhibited a sensitivity of around 87/db/w/m which is basically Vifa's published sensitivity for this driver.
It's after the baffle step point is reached that the output rises to the +6db level.
You can view the full set of graphs at
http://www.t-linespeakers.org/tech/filters/Jeff/.

MADINOZ

If you read the Seas data sheet completely you will see it it
measured at 0.5m but driven by 2V to be equivalent to 1m.
0dB is 60dB as specified on the sheet.

It is the graph you mention that has no calibration.

I'll also note all the other graphs in the article show results
which plainly agree with the real situation as outlined in my
previous post.

sreten.

[rabbitz]

I'm trying to come to terms with baffle step compension using a 0.5 woofer at the back of the cabinet.

If we have a baffle of 200mm wide, a common size, then the baffle step frequency would be 575Hz. Is this the centre of the baffle step or where it starts to fall off? What is the effect on frequencies either side? That is, does the step go out to the woofer bottom end roll off and then rise again at the midrange at some frequency above 575Hz, a multiple of 575Hz or 575Hz + xHz???

If you used a 0.5 woofer, does it have to be the same as the front woofer with the same F3 or can it be a smaller woofer that has a higher F3 which rolls off at 575Hz + xHz at 6dB?

If you use a 0.5 woofer in parallel with the front woofer, I take it the sensitivity of the speaker remains the same as you are only using the 0.5 to cover the baffle step frequencies. Whereas if you used 2 woofers that cover the same range then the sensitivity goes up. So I assume the L-pad for the tweeter would not be effected.

Questions, questions...... have read a lot on the subject using passive filter compensation as well as using a 0.5 woofer but am having trouble puting the puzzle pieces together.

I'm lost in my own post........ I hope you understand what I mean.

[sreten]

Hi Rabbitz,

An MTM 2 way with full baffle step compensation will have the
same tweeter level as a 2.5 way.

2.5 way forces you to have 6dB BSC, whilst a MTM 2 way can
have less BSC, thus it can have higher tweeter sensistivity.

THE baffle step is approximated to be a slope either side of the
centre frequency, how to estimate the centre frequency ?, I've
seen various ratio used.

The F3's of the two woofers don't have to be the same, see here :

http://home.hetnet.nl/~geenius/Auriga.html

The graphs should help somewhat,

sreten.

(P.S. if you place the extra bass unit on the back of the box
- do you mean like here ?

http://www.t-linespeakers.org/projects/tlB/intro.html

This gets confusing as the monopole and bipole cases are different.)

[Svante]

Originally posted by sreten
Hi Rabbitz,

2.5 way forces you to have 6dB BSC, whilst a MTM 2 way can
have less BSC, thus it can have higher tweeter sensistivity.

Hmm... You *could make it "2.7 way", I mean just *lower* the level of one driver towards higher frequencies. The first order filter, that would be just an inductor in the 2.5 case would now be an inductor in parallel with a resistor. But then, we are almost at the "MTM-with-BSC" design... Oh well...

[sreten]

Originally posted by Svante


Hmm... You *could make it "2.7 way", I mean just *lower* the level of one driver towards higher frequencies. The first order filter, that would be just an inductor in the 2.5 case would now be an inductor in parallel with a resistor. But then, we are almost at the "MTM-with-BSC" design... Oh well...

Hi Svante, ( I never get away with my generalisations...)

True, but your asking for trouble with the usual TMM
2.5 way set up allowing midrange thorough to the
lower driver. In a MTM 2.5 & a bit way this is less of
an issue but it does complicate the mid/treble c/o.

For the MTM 2.5+way :
If you are not careful the 0.5+ way driver will have more
output above the c/o frequency than the main unit, if you
only add a parallel resistor.
So you need to c/o this driver as well if you want it to work well.
Getting very complicated compared to the 2-way MTM case.

sreten.